As the light-emitting efficiency is increased and the cost of manufacturing is decreased, the dream for solid lighting device to replace the traditional lighting device may come true in recent years. Currently, the inner light-emitting efficiency of the light-emitting diode is about 50% to 80%, but a part of the light may be absorbed by the electrode or the light-emitting layer so the total light-emitting efficiency is degraded. Therefore, the mirror layer under the electrode has been provided to solve the problem. When the route of the light extracted from the light-emitting layer is blocked by the electrode, the mirror can reflect but not absorbs the light.
When the electrical current is applied into the light-emitting diode and flows through the electrode, the material of the mirror layer may be migrated to the electrode. The migration of the material of the mirror layer causes the increase of the resistance of electrode and lowers the light-emitting efficiency of the light-emitting diode. So a barrier layer 11 is provided to prevent the migration of the material of the mirror layer 12 as shown in FIG. 1. Nevertheless, in recent years, as the light from a single chip is required to emit more, the electrical current applied to the light-emitting diode is increased, and the large electrical current may destroy the barrier layer 11.
The conventional mirror layer 12 is formed of Al or Ag, and the conventional barrier layer 11 is formed by a single metal which can defense the migration of the mirror layer 12, such as Ti or Mo. But, it is not easy to form a barrier layer 11 which has a uniform thickness on the mirror layer 12. When there is an electrical current passes the mirror layer 12, the metal of the mirror layer 12, such as Al or Ag, would destroy the thinner portion of the barrier layer 11. Therefore, the barrier layer 11 formed by two different metals is preferred. The second metal layer can complement the thinner portion of the first metal layer to enhance the defense of the barrier layer 11. But, if a high electrical current flows through the electrode, the barrier layer 11 formed by two different metals is usually still destroyed by the material of the mirror layer 12. FIG. 2A shows the destroyed electrode 4 with a barrier layer of two different metals under a 120 mA current supplied for 97 hours. FIG. 2B shows the barrier layer 11 has been destroyed. The mirror layer has migrated and been alloyed with the other portion of the electrode 4, and, thus, the conductivity of the electrode 4 substantially decreases.